1. The Field of the Invention
This invention relates to a method for preparing a copolymer from a conjugated diene monomer and a vinyl aromatic monomer. More particularly, this invention relates to a method for preparing a copolymer useful for the fabrication of tires by coupling the active ends of the copolymer with a multi-reactive polysiloxane and subsequently modifying the remaining active ends of the copolymer with a mono-reactive amine compound.
2. Related Prior Art
Regarding a conjugated diene polymer useful as a rubber material for tires, such as a styrene-butadiene copolymer, the conjugated diene polymer prepared in the presence of an organolithium catalyst in an organic solvent is controllable in regard to molecular structure such as the vinyl structure of conjugated diene, the styrene content, the styrene block percentage, and so forth, which cannot be controlled by emulsion polymerization, providing required properties as a material for tires. The use of the rubber prepared by the solution polymerization for fabrication of tires guarantees very excellent properties of tire products, such as rolling resistance and wet traction, relative to the use of a styrene-butadiene rubber (SBR) prepared by emulsion polymerization. In addition, the use of an organolithium catalyst for polymerization introduces different functional groups to the ends of the molecule to enable the control of cold flow at the ambient temperature and also to increase the compatibility with a reinforcing material such as carbon black or silica, thus enhancing tread wear, reducing rolling resistance and increasing wet traction.
Among the conventional methods to increase the compatibility of the conjugated diene polymer prepared by solution polymerization with carbon black and silica, U.S. Pat. No. 4,555,548 discloses a method for increasing the compatibility of rubber with carbon black by modifying the ends of the molecule with an amine compound such as amino benzophenone to produce a rubber superior in dynamic properties and mechanical properties to the existing rubbers. However, the rubber prepared by this method has a poor processability in mixture and a high cold flow, which is an important factor of storage stability, causing a problem after a long-term storage. The use of this rubber for a tire containing silica as a reinforcing material (hereinafter, referred to as “silica tire”) may result in poor compatibility with silica to deteriorate the mechanical properties and the dynamic properties (e.g., rolling resistance and wet traction) of the tire. Consequently, the use of the rubber does not have any merit in the fabrication of silica tires.
U.S. Pat. No. 6,329,467 describes a method of using a mixture of tin tetrachloride and silicon tetrachloride as a coupling agent for the purpose of reducing the processability and the rolling resistance of the carbon black composition. The polymer prepared by this method has no functional group contributing to the compatibility with a reinforcing material and hence a poor compatibility with the reinforcing material in the silica composition, making it difficult to prepare a rubber suitable to the silica tire. Particularly, this method deteriorates the properties of the silica composition because the tin tetrachloride compound breaks the bonds between tin and the polymer by stearic acid added as a vulcanizing accelerator in the mixing step.
In addition, U.S. Pat. No. 6,133,388 discloses a method of maximizing the compatibility with a reinforcing material by modifying both ends of the molecule into functional groups. However, solution viscosity rapidly increases during a polymerizing procedure employing such an end modifying method, thus it is difficult to apply to a commercialized process. This polymer having a linear molecular structure is very disadvantageous to apply concerning a compound processability and storage stability point of view.
Korean Patent Laid-open Nos. 94-019797 and 94-019795 disclose methods for enhancing the compatibility with silica used as an inorganic reinforcing material, which methods involve dispersing polydimethylsiloxane in a styrene-based resin composition to remarkably enhance tread wear and cold shock resistance. However, the polymer composition prepared by dispersion of polydimethylsiloxane has no covalent bonds between the polydimethylsiloxane and the organic polymer resin, readily exhibiting phase separation, and contains neither a polar group nor a hydrophilic group in the polydimethylsiloxane, resulting in a low affinity to the inorganic reinforcing material and hence a deterioration of the compatibility with the inorganic reinforcing material.
Consequently, the modification of the polymer that is a highly reactive organic material is necessary for enhancing the compatibility of organic and inorganic materials in the development of such an organic/inorganic composite material, and many studies have been made on the polymer modification in a physical way. For example, Korean Patent Laid-open No. 95-704405 discloses a method of polymerizing hexamethylcyclotrisiloxane at the end of a living polymer block. This method may solve the problem in regard to phase separation into polysiloxane and the organic layer but still provides a high cold flow of the polymer and a deterioration of mixing processability as mentioned in the preceding methods.
In addition, in an attempt to increase the miscibility of the organic and inorganic materials, the ends of an anionic living polymer can be reacted with ethylene oxide (J. Polym. Sci., Part A: Polym. Chem., 26, 2031 (1988)), diphenylethylene (J. Polym. Sci., Part A: Polym. Chem., 30, 2349 (1992)), or N-(benzylidene)-trimethylsilylamine (Makromol. Chem., 184, 1355 (1983)). However, this method has a limitation in acquiring sufficient compatibility with an inorganic reinforcing material.
Accordingly, there is a demand for a novel polymer excellent in miscibility with inorganic fillers used in the fabrication of tires, while improving the shortcoming of the existing polymers.